This invention relates to induction heating coils in general and in particular to such heating coils of small size which can produce high heats for use in thermoplastic welding operations using metallic susceptors to generate heat by eddy current induction.
Induction heating coils used in thermoplastic welding of non-metallic composite parts are often required to be of small size in order to allow their use in restricted places such as those found in aircraft structures. While a small size is desirable, it is often required that the temperature needed to make an effective thermoplastic weld requires relatively high power inputs to the coil. Under these conditions it is necessary to provide outside cooling to the coil to prevent overheating. In the prior art, cooling has been provided by winding the coil on square or round copper tube and running water or other coolant through the tube when the coil is excited. When such tubing is used to support a coil it is nearly impossible to build a small coil of less than one inch in diameter. To construct such a small coil with a tubular core allows only a few turns of wire to be placed around the coil. With so few turns, perhaps as few as three or four, the inductive reactance of the coil is necessarily low making it difficult to match the coil impedance to an RF power supply necessary to excite the coil at the proper frequency. Also, due to skin effect, only a portion of the copper of the windings is effective in carrying the required current thus making the coil losses high and the coil inefficient.